The present invention relates to an electric power steering system for a motor vehicle, and more particularly to a system for controlling a motor in accordance with the turning of a steering wheel.
The electric power steering system has a torsion torque sensor for measuring torsion torque in a steering shaft. The system is adapted to produce an assist signal to control the rotational direction and torque of a motor in a dependency on a torque signal from the sensor (Japanese Patent Application Publication 45-41246). The response of the steering system and the returnability of the steering wheel may deteriorate because of the inertia of the motor, the friction of a reduction gear train provided in the system, and other factors.
There has been proposed a control system which may prevent the above-described disadvantages (U.S. Pat. No. 4,664,211). The system is provided for producing an assist signal dependent on the torsion torque signal from the torsion torque sensor and on the vehicle speed, a return torque signal depending on an output signal of a steering angle sensor for returning the steering wheel to a neutral position, and a damping signal depending on the angular speed of a steering wheel. These signals are added for controlling the motor.
In such a system, if the damping force is set to a large value for stabilizing the steering wheel at high vehicle speed driving, the convergency of the steering wheel at low speed driving deteriorates. To the contrary, if the damping force is set to a value for improving the convergency at the low speed driving, a long converging time occurs at high speed driving.